Wow! This got a lot of attention.
The problem I have with ice packs is our freezer is very small and very full most of the time. The interior is roughly the size of a 1990's microwave oven, but divided into two shelves. We can't store a roaster chicken in it. We don't use ice cubes because there is no space for ice cube trays.
An ice pack for a back or a hot water bottle is roughly the size of an ice cube tray, if it's foldable.